(a) Field of the Invention
The present invention relates to a shift control method for a vehicle equipped with an electronic throttle control (ETC) system and a gasoline direct injection (GDI) system which prevents the occurrence of shock generated during tip-in.
(b) Description of the Related Art
Shock is generated during tip-in using a conventional transmission. In FIG. 1, line 1 is a waveform of an output of a throttle valve position sensor for a vehicle that is not equipped with an ETC system. In such a vehicle, the throttle is indexed with an accelerator pedal. Line 2 in the drawing is a waveform of an output of a throttle valve position sensor for a vehicle having an ETC system. Nt in the drawing represents turbine rpm, Ne is engine rpm, and T is torque. As shown by line 2, in a vehicle having an ETC system, the throttle valve opening varies linearly in response to the abrupt operation of the accelerator pedal. That is, when the accelerator pedal is abruptly depressed, the throttle valve opening increases at a uniform rate.
Region A in FIG. 1 indicates a state in which the accelerator pedal has been released such that the vehicle operates in a lean-burn mode. At this time, in a vehicle not equipped with an ETC system, if the driver again depresses the accelerator pedal, the throttle valve position sensor generates a throttle valve opening voltage as shown in line 1 according to the abrupt increase in throttle valve opening. That is, in a vehicle not having an ETC system, the throttle valve is indexed with the accelerator pedal. On the other hand, in a vehicle having an ETC system, in order to prevent the abrupt increase in torque as a result of the abrupt increase in throttle valve opening, the ETC system delays the response in the throttle valve opening such that the resulting throttle valve opening voltage is increased at a predetermined rate as shown in line 2.
In the conventional vehicle, the vehicles runs in a lean-burn mode when the accelerator pedal is not being depressed (region A), and runs in a normal mode when the accelerator is depressed (region B). The transition to the normal mode from the lean-burn mode occurs when the output of the ETC exceeds, for example, 0.7V, that is when the output of the ETC increases 0.2V from the 0.5V output of the lean-burn mode. However, when converting to the normal mode form the lean-burn mode, engine rpm Ne abruptly increases to a level greater than the turbine rpm Nt. Accordingly, the torque T undergoes a backlash as shown by region C in FIG. 1, thereby resulting in the generation of shock.
The present invention has been made in an effort to solve the above problems.
It is an object of the present invention to provide a shift control method which reduces shock generated as a result of a delay in the increase in engine rpm when converting from a lean-burn mode to a normal mode during tip-in.
To achieve the above object, the present invention provides a shift control method for an automatic transmission comprising the steps of converting to a lean-burn mode from a normal mode if a throttle voltage is less than a first predetermined value; determining if tip-in has occurred in the lean-burn mode using accelerator pedal signals; increasing throttle valve opening if it is determined that the throttle voltage has increased by a second predetermined value such that engine rpm is increased; determining if a difference between engine rpm and turbine rpm is greater than a predetermined number of rpm; and converting from the lean-burn mode to the normal mode if engine rpm is greater than turbine rpm by the predetermined number of rpm.
The shift control method may further comprise the step of retarding the converting operation from the lean-burn mode to the normal mode until the difference between the engine rpm and the turbine rpm becomes greater than the predetermined number of rpm after the throttle voltage has increased by the second predetermined value.
The shift control method may further comprise the step of controlling the throttle valve to be opened in response to a maximum stroke of the accelerator pedal after the step of converting from the lean-burn mode to the normal mode.